1. Field of the Invention
The present invention relates to a raindrop sensor, which is specifically suitable for a vehicle wiper automatic control device.
2. Description of Related Art
Conventionally, a raindrop sensor for detecting raindrops, which attach to a vehicle windshield and the like, is known (see Unexamined Japanese Patent Publication No. 2001-66246 corresponding to U.S. Pat. No. 6,507,015). The raindrop sensor shown in the Unexamined Japanese Patent Publication No. 2001-66246 is mounted on an interior wall of the windshield 200, and optically detects raindrop attachment as shown in FIG. 9. The raindrop sensor mainly includes a light guide body 140, planoconvex lenses 120, 130, planoconvex lens segments 150, 160, light-emitting elements 100, 110 and a light-receiving element 170.
The light guide body 140 includes inclined planes, where the planoconvex lenses 120, 130 and the planoconvex lens segments 150, 160 are formed. The planoconvex lenses 120, 130 face the light-emitting elements 100, 110. The planoconvex lens segments 150, 160 face the light-receiving element 170. A plurality of inclined planes is formed on an input side of the light guide body 140. Each of the planoconvex lenses 120, 130 is formed on a corresponding inclined plane. On the contrary, a plurality of inclined planes is formed on an output side of the light guide body 140. The planoconvex lens segments 150, 160 are formed on the corresponding plurality of inclined planes. The planoconvex lens segments 150, 160 are generated by dividing a planoconvex lens into several lens segments, a number of which is equal to that of the plurality of inclined planes.
Each light from the light-emitting elements 100, 110 is collimated to form a collimated light beam (shown by a two-dot chain line) through the planoconvex lenses 120, 130 on the input side. A predetermined region on the windshield 200 is irradiated with the collimated light beam. This region is defined as a raindrop-sensing region.
Reflecting light at the raindrop-sensing region is converged through the planoconvex lens segments 150, 160 on the output side. Then, the light-receiving element 170 receives the reflecting light to detect a raindrop amount at the raindrop-sensing region. In a structure of the above described conventional art, as shown in FIG. 9 the light from the light-emitting elements 100, 110 is applied to the planoconvex lenses 120, 130 in the input side. However, a part of the planoconvex lens 130 (around a dividing surface 180) does not receive the light from the light-emitting element 100. Therefore, an amount of light that the light guide body 140 receives is decreased, resulting in decreasing an amount of the light, which travels from the planoconvex lens 130 to the windshield 200 (shown as a shaded area in FIG. 9). As a result, accuracy for detecting raindrops is degraded at the part of the raindrop-sensing region, which receives the decreased amount of light.
Around a dividing surface 190 on the output side, in converging the reflecting light through the planoconvex lens segments 160, 150, the dividing surface 190 on the output side and the planoconvex lens 150 may limit the reflecting light from being converged in some cases. As a result, an amount of light, which travels from the windshield 200 to the planoconvex lens segment 150 (shown as a shaded area in FIG. 9), may be decreased.